Researchers from Harvard and Stanford universities published three studies today showing that substances in the blood of young mice rejuvenate the muscles and brains of older ones. A previous Harvard study suggests the same happens to their hearts.

"Their fur looks better, they groom better, they seem to do overall better," said Tony Wyss-Coray, the lead scientist on the Stanford study, to be published in the journal Nature Medicine. "To us it's just so surprising, that something so simple has dramatic effects on every tissue in the body that's been looked at."

Whether the same process will work in people remains a mystery, but the researchers are curious and hopeful.

Wyss-Coray, a professor of neurology, said he's planning to start a small clinical trial to see whether Alzheimer's patients can tolerate and are helped by the plasma of healthy younger people.

The Harvard researchers are considering trials in Alzheimer's, heart disease and other aging-related conditions, said Lee Rubin, a neuroscientist and professor in the school's Department of Stem Cell and Regenerative Biology. Rubin said he's also interested in understanding whether this regenerative ability would help people with genetic diseases, such as the rare neural disease, spinal muscular atrophy, that he researches.

The mice were studied by surgically attaching a younger mouse to an older one and having them share the same blood supply.

"Exposing young mice to old recharges their brain and now they remember better and behave in many ways like younger mice," Wyss-Coray said.

The two Harvard papers, both coming out in the journal Science later this week, tested a protein found in abundance in the blood of younger mice and less so in older ones, called growth differentiation factor or GDF11. Human blood also has the same protein.

In the second Harvard study, researchers led by stem-cell biologist Amy Wagers showed that boosting GDF11 in the blood of older mice allowed them to run faster on a treadmill and increased their ability to recover from an in injury. It's not clear how much GDF11 people would need to get a similar effect, if that were even possible, Wagers said.

The three researchers said they haven't seen any side effects in the mice, though they have not yet done studies designed to look for potential problems.

Future research will explore whether people who live extra long lives have more GDF11 or other proteins in their blood that might contribute to their longevity, Wagers said.

Researchers are also unsure whether using GDF11 is the best way to capitalize on the restorative ability of blood. GDF11 might be the fountain of youth, Wagers said, or "maybe it's a drop in the fountain."

Dan Perry, the founder of the Alliance for Aging Research, a non-profit, said he's impressed with all the animal research but ready for scientists go further.

With the aging of the baby boomers, he said, "we need to get about funding this research for human use in time to meet the tsunami of age-related diseases that are headed our way."